1. Field of the Invention
The present invention relates generally to gas turbine engine blades and, more particularly, to a turbine blade squealer tip having air cooling holes through the tip end cap and arranged contiguous with the tip interior wall surface adjacent at least one of the pressure and suction sides of the blade.
2. Description of the Prior Art
It is well known that a reduction in gas turbine engine efficiency results from leaking of hot expanding combustion gases in the turbine across a gap between rotating turbine blades and stationary seals or shrouds which surround them. The problem of sealing between such relatively rotating members to avoid loss in efficiency is very difficult in the turbine section of the engine because of high temperatures and centrifugal loads. One method of improving the sealing between the turbine blade and shroud is the provision of squealer type tips on turbine blades. A squealer tip is composed of a continuous tip wall of relatively small height surrounding and projecting outwardly from an end cap on the outer end of a turbine blade which closes a cooling air plenum in the interior of the blade.
During operation of the engine, temperature changes create differential rates of thermal expansion and contraction on the blade rotor and shroud which may result in rubbing between the blade tips and shrouds. Centrifugal forces acting on the blades and structural forces acting on the shrouds create distortions thereon which may also result in rubs. Such rubbing interference between the rotating blade tips and surrounding stationary shrouds causes heating of the blade tips resulting in excessive wear or damage to the blade tips and shrouds. It is, therefore, desirable to cool the blade tips. However, in the case of squealer type blade tips, heating produced by such rubbing interference is actually augmented by the presence of a cavity defined by the end cap of the blade and the peripheral wall of the squealer tip. Therefore, squealer type blade tips, though fostering improved sealing, actually require additional cooling.
Because of the complexity and relative high cost of replacing or repairing turbine blades, it is desirable to prolong as much as possible the life of blade tips and respective blades. Blade tip cooling is a conventional practice employed for achieving that objective. The provision of holes for directing air flow to cool blade tips is known in the prior art, for instance as disclosed in U.S. Pat. No. 4,247,254 to Zelahy, and have been applied to squealer type blade tips as disclosed in U.S. Pat. No. 4,540,339 to Horvath.
Turbine engine blade designers and engineers are constantly striving to develop more efficient ways of cooling the tips of the turbine blades to prolong turbine blade life and reduce engine operating cost. However, cooling air used to accomplish this is expensive in terms of overall fuel consumption. Thus, more effective and efficient use of available cooling air in carrying out cooling of turbine blade tips is desirable not only to prolong turbine blade life but also to improve the efficiency of the engine as well, thereby again lowering engine operating cost.
One recent improvement in the blade tip cooling holes to achieve enhanced cooling of blade tips and thereby improved engine operating efficiency and blade tip life is disclosed in U.S. Pat. No. 4,893,987 to Lee et al, which is assigned to the assignee of the present invention. This improvement, which is described later in the prior art section of the detailed description, constitutes a positive step toward better blade tip cooling. However, there is a continuing need for additional improvements in this area of turbine blade design.